Timing Advance is a MAC CE that is used to control Uplink signal transmission timing. Network (eNodeB in this case) keep measuring the time difference between PUSCH/PUCCH/SRS reception and the subframe time and can send a 'Timing Advance' command to UE to change the PUSCH/PUCCH transmission to make it better aligned with the subframe timing at the network side. If PUSCH/PUCCH/SRS arrives at the network too early, network send a Timing Advance command to UE saying "Transmit your signal a little bit late", If PUSCH/PUCCH/SRS arrives at the network too late, network send a Timing Advance command to UE saying "Transmit your signal a little bit early".

 Timing Advance是MAC层的控制单元，用于控制上行信号发送时机；

网络侧（eNodeB）一直监测PUSCH/PUCCH/SRS上接收（的信息）和下行子帧（发送的信息）时间差，用“Timing Advance”命令通知UE在PUSCH/PUCCH上调整信号发送时机，以便与网络侧（子帧）时间一致；

如果PUSCH/PUCCH/SRS信号到达的时间过早，网络下发“延迟发送信号“命令；

如果PUSCH/PUCCH/SRS信号到达的时间过晚，网络下发“提早发送信号“命令；

MAC PDU for Timing Advance is as follows. It is one byte data and the first two bits are reserved and set to be always 0. The remaining 6 bits carries Timing Advance command value ranging from 0 to 63.

Timing Advance 在MAC层PDU为1byte数据，其中前2bits保留，全设为0，           剩下的6bits承载Timing Advance命令，取值范围：0~63；

As you see in the following figures, for Rel 8,9,10 there is no special tag for each component carrier, meaning that even in Carrier Aggregation single Timing Advanced value apply to all the component carriers. But in Rel 11, the first 2 bits are allocated to indicate whether the value is for PCC or SCC. If TAG id is 0, it means it is for PCC.

LTE在R8，9，10版本中对载波没有区别标识，即便在载波聚合（CA）中每个载波都相同；

从R11版本，前2bits被用来区分PCC或SCC载波；如果标识为0，指示载波为PCC;

< 36.321 Rel 8,9,10 - Figure 6.1.3.5-1: Timing Advance Command MAC control element >

< 36.321 Rel 11 - Figure 6.1.3.5-1: Timing Advance Command MAC control element >

Then how to translate each value of TA(Timing Advance) value to physical 'time' delay or advance value. It is described in detail in 36.213 4.2.3 Transmission timing adjustments. Simply put, the UL transmit timing is controlled by following equation.

 在36.213 4.2.3中对TA(Timing Advance)与物理上的时间（提前or延后）换算有专门描述；上行发送时间计算公式如下：

UL Transmission Time = (UL Transmittion Time for Previous subframe) + (TA value - 31) x 16 samples.

上行发送时间=上行前一帧发送时间+（TA值-31）*16（采样点）

where 1 sample is about 0.033 us and 16 samples is about 0.52 us.

1个采样点约为0.033us,16个采样点约为0.52us; 

By this calcuation, you can see that the maximum timing change by single TA value (0 or 63) is about 16.7 us (I hope my calculation is right. please let me know if this calculation is wrong).

根据公式：TA（0 or 63）最大值为16.7us 

Example 1 > Timing Advance in a Live Network (in the field) 

Following plot is from the data captured by a drive test tool Azenqos Drive Test tool (AZQ Android). I got the log captured by the tool and exported the data as csv file and then plot it on Microsoft Excel. The map displaying the path of the measurement shown at the bottom is the one automatically created by AZQ reporting tool. It is hard to interpret the exact meaning of each points unless you have the exact location of eNB displayed on the map and the distance between UE and the eNB is recorded, but at least you would see some interesting pattern of TA value changes as the UE moving along the path getting closer to and farther away from the serving eNB. 

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LTE中的TA.docx